1. Field of the Invention
The present invention relates in general to a display apparatus, and more particularly, to a display apparatus having a driving circuit for a high voltage flyback transformer (FBT) incorporating a deflecting function and which is capable of obtaining a high resolution display.
2. Description of the Related Art
As shown in FIG. 5, a high voltage FBT driving circuit incorporating a deflecting function includes a high voltage detector 22 to detect an output voltage of an FBT 21, a high voltage compensator 23 to output a high voltage compensating signal based on the voltage detected by the high voltage detector 22, a DC voltage controller 25 (hereinafter referred to as a “B+ controller”) to adjust a DC voltage according to the high voltage compensating signal and to supply the adjusted DC voltage to a primary coil T1 of the transformer 21. A horizontal deflector 29 generates a horizontal deflecting pulse using the B+ voltage output from the B+ controller 25, and a diode modulator 31 adjusts a horizontal size of a screen.
To operate the high voltage FBT driving circuit incorporating a deflecting function, a commercial AC voltage is converted into a DC voltage through a DC converter (not shown), stabilized into the B+ voltage through the B+ controller 25 according to the high voltage compensating signal, and then supplied to the primary coil T1 of the FBT 21. At this time, a secondary coil of the FBT 21 outputs a high voltage to an anode of a picture tube (not shown).
Herein, in order to keep the screen of the picture tube at a fixed size regardless of a horizontal synchronous frequency, the high voltage should be constant. The constant high voltage is due to the screen size of the picture tube decreasing as the high voltage increases. Thus, the B+ controller 25 supplies the B+ voltage based on the horizontal synchronous signal to the FBT 21 so as to keep the high voltage constant.
Referring to the deflection pulses of FIGS. 4A and 4B, the change of the B+ voltage according to the horizontal synchronous frequency (resolution) will be described herein below. The B+ controller 25 controls the B+ voltage according to the high voltage compensating signal applied from a primary coil T3 to the high voltage compensator 23 so as to keep the deflection pulse applied from the primary coil T1 to the horizontal deflector 29 at a fixed amount. That is, even though the horizontal synchronous signal changes according to the resolution, as long as the size of the deflection pulse applied from the primary coil T1 to the horizontal deflector 29 is constant, the high voltage is constant. This is accomplished by allowing the B+ controller 25 to control the B+ voltage level in order to make areas “A” and “B” be equal each other.
On the other hand, a flyback time of the deflection pulse generated from the horizontal deflector 29 is determined by a resonance cycle of a horizontal-deflection yoke H-DY and a resonance capacitor Cr. Herein, the flyback time is constant regardless of the horizontal synchronous frequency (resolution), and a video signal is output from a video card (not shown) at 80 percent of total horizontal cycles so that a raster margin at a high resolution is smaller than at a low resolution.
In the conventional high voltage FBT driving circuit incorporating the deflecting function, the highest resolution is obtained at a mode in which the driving circuit is determined such that the lowest B+ voltage is output at the lowest resolution and the minimum raster margin is secured. At this time, the highest resolution performed by this circuit has a horizontal frequency of 70 kHz.
To make the above horizontal frequency higher by using the conventional driving circuit, the raster margin of the high resolution must be enlarged. Further, to enlarge the raster margin at the high resolution, a resonance capacitor switching circuit may be added thereto to decrease the capacitance of the resonance capacitor Cr.
However, if the capacitance of the resonance capacitor Cr is decreased in order to secure the raster margin of the high resolution, the flyback time is shortened so that the B+ voltage adjusted by the B+ controller 25 is dropped below a limit level when the high resolution changes to the low resolution. Further, the capacitor switching circuit is expensive and unstable. Further, the highest resolution of the driving circuit using the capacitor switching circuit is not very high.